1. Field of the Invention
The present invention relates to an improved switch. The present invention also relates to an improved high-voltage device utilizing the improved switch and having a high continuous current rating. More specifically, the present invention relates to an improved high-voltage circuit-protection device, and to current-limiting or non-current-limiting high-voltage fuses, which, along with the improved switch, constitute a portion of the improved device, both types of fuses more conveniently achieving a higher continuous current rating than possessed by known fuses. The improved device is reliable in operation, convenient and economical to manufacture, and partially reusable, thereby reducing replacement and maintenance costs. The present invention is an improvement of the invention disclosed in commonly assigned abandoned U.S. patent application, Ser. No. 972,650, filed Dec. 21, 1978 in the name of Otto Meister.
2. Brief Discussion of the Prior Art
Fault currents (used herein to mean all undesirable over-currents), impress rather high thermal and mechanical stresses on high-voltage electric systems and on apparatus used in such systems. The severity of the thermal stresses is known to be generally proportional to the product of (1) the square of the fault current, and (2) time--i.e., I.sup.2 t. The severity of the mechanical stresses is generally proportional to the square of the peak or crest value achieved by the fault current. Thermal stresses are generally manifested in the burning of, or other thermal damage to, lines, cables, internally faulted transformers and other equipment attached to electrical systems. The mechanical stresses are manifested in the deformation of bus work and switches and in damage to items, such as transformer or reactor coils, due to the extremely high magnetic forces generated by the fault current.
Circuit switchers and circuit breakers are well-known devices for protecting high-voltage electrical systems and apparatus connected therein. These devices have high continuous current ratings, as well as substantial fault-current-interrupting capabilities. Expulsion fuses, which are also used for high-voltage circuit protection, have somewhat lower continuous-current ratings than breakers and circuit switchers. To the present, none of these devices, regardless of continuous-current rating, possess the consistent ability to limit, in all cases, both fault current peaks and I.sup.2 t to low values. That is, while these devices do interrupt current, they are usually not able to limit current peaks or I.sup.2 t until interruption occurs. Thus, if such devices do happen to limit current peaks or I.sup.2 t to low values, it is because interruption occurs by happenstance a very short time after initiation of the fault current. For these devices to be rendered consistently capable of interrupting fault currents very shortly after initiation thereof is an expensive proposition. Accordingly, although these devices may well protect the overall high-voltage system from severe, widespread damage, some damage may nevertheless result to either the system or to the apparatus therein due to the fact that the fault current peaks and I.sup.2 t may achieve substantial magnitudes prior to current interruption.
Current-limiting fuses of the so-called silver-sand variety and other current-limiting devices are well known expedients for limiting the magnitude of fault currents. See the following, commonly assigned U.S. Pat. Nos. 4,063,208 to Bernatt; 4,057,775 to Biller; 4,035,753 to Reeder; 4,028,656 to Schmunk and Tobin; 4,011,537 to Jackson and Tobin, and; 4,010,438 to Scherer. Compared to circuit-switchers, circuit breakers and expulsion fuses, current-limiting fuses both interrupt fault currents and limit peak fault current and I.sup.2 t to more tolerable levels. These tolerable levels of peak fault current and I.sup.2 t are lower than the values which are usually reached when circuit switchers, circuit breakers, or expulsion fuses are used. These lower values of peak fault current or I.sup.2 t are often termed the "let-through current" or, simply "let through." Current-limiting fuses, therefore, are designed to (1) interrupt fault currents and (2) limit the peak fault current and I.sup.2 t to tolerable magnitudes, thereby minimizing thermal and mechanical stresses. However, as is well known, current-limiting fuses, particularly at higher voltages, have low continuous-current ratings which impose limitations on the applicability thereof.
As electrical systems have expanded, and electric consumption has increased, continuous current in such systems has also increased. Because of the low continuous-current rating of conventional silver-sand current-limiting fuses, such fuses have had only limited applicability in high-voltage systems. The low continuous-current rating of current-limiting fuses is apparently inherent; known current-limiting fuses cannot meet both requirements of low let-through and high continuous-current rating without some modification or the addition of special apparatus. Further, fault-current levels have begun to exceed the capability of existing switchgear. If, in order to avoid the occurrence of increased fault-current levels, electrical systems are arranged so that they contain individual sections having low available fault currents, or, if current-limiting reactors, high impedance transformers or the like are used, certain disadvantages may nevertheless result. For example utilization of, sectionalization and the use of current-limiting reactors are uneconomical and may render voltage regulation difficult to achieve. These techniques also usually produce an over-abundance of idle reserve in the electrical system. Thus, unless an economical and reliable current-limiting fuse having a high continuous-current rating becomes generally available, the only solution--a costly one--to solve the problem of increased fault-current levels is to replace existing switchgear with gear having higher fault and overcurrent withstand capabilities and higher interrupting capabilities.
Accordingly, the fault-limiting properties of current-limiting fuses are so desirable that they have been, and remain, the subject of great interest.
Approximately twenty years ago, a device, sometimes referred to as an "I.sub.s -Limiter," was developed by Calor-Emag Corporation (now a division of Brown Boveri, West Germany). The I.sub.s -Limiter is constructed with a high-continous-current-capacity main conductive path which is electrically paralleled with a more or less standard current-limiting fuse. The current-limiting fuse may be of the well-known silver-sand type having a silver fusible element surrounded by a fulgurite-forming arc-quenching medium, such as silica or quartz sand. The main conductive path of the I.sub.s -Limiter includes a so-called "bursting bridge" which, upon detonation of a chemical charge contained therewithin in response to a fault current, renders the main conductive path discontinuous and rapidly transfers or commutates the current flowing through the main conductive path of the current-limiting fuse.
The bursting bridge is comprised of a pair of tube sections, each open at one end and containing longitudinal slots over the majority of their length. The open ends of the tube sections are joined along a brazed, weak interface to enclose the chemical charge. Detonation of the chemical charge breaks the weak interface, blowing up the bursting bridge and bending fingers defined between the solts of each tube section out and back in a "banana peel" configuration; this renders discontinuous the main conductive path. See U.S. Pat. No. 2,892,062 to Bruckner, et al. This discontinuity in the main conductive path transfers or commutates the current to the current-limiting fuse, which current is then interrupted in a conventional manner common to silver-sand current-limiting fuses. The chemical charge is detonated by means of a pulse transformer, or other electronic device, contained in one of two insulators which mounts the combination of the current-limiting fuse and the main conductive path, each housed in its own individual insulative housing.
When the bursting bridge is blown apart, an arc forms between the tube sections. The arc voltage is, sometime thereafter, sufficiently high to commutate the current to the fusible element so that interruption in the current-limiting fuse may occur. If not properly fabricated, the bursting bridge may not fully open. Further, it has been found that the gap between the bent-back fingers of the tube sections may be ionized by hot ignition products, mostly gaseous, due to detonation of the chemical charge. Such ionization permits the arc to persist and/or lowers the arc voltage, thus slowing or preventing commutation of the current to the current-limiting fuse. It has also been found, however, thay by careful design and construction the dielectric strength across the gap usually recovers, or at least usually increases rather quickly, after about 200 microseconds. Therefore, the fusible element of the current-limiting fuse portion of the I.sub.s -Limiter must be so designed and constructed at to (a) overlap the "dead time" of the bursting bridge until the 200 microsecond time passes, and then (b) limit and interrupt the current. Following the initial 200 microseconds, voltage stress across the gap has been found to be rather low, due to the lower resistance of the fusible element as compared to that of the gap. Thus, the I.sub.s -Limiter is a current-limiting device combining a fast-acting switch having a high-continuous-current capability but poor current-interrupting capability, with an electrically parallel current-limiting fuse having a low-continuous-current capability but high current-limiting and interrupting capability.
Several disadvantages of the I.sub.s -Limiter should be noted. First, the current-limiting fuse and the main conductive path form two separate elements in their own separate housings. This arrangement is not only somewhat clumsy and difficult to manipulate during replacement or initial placement, but increases material costs due to the duplication of certain elements, such as housings, end ferrules, conductors, and the like. Second, commutation of the current flowing through the main current path to the current-limiting fuse may be slower than it might otherwise be, because the inductance of the main conductive path and current-limiting fuse combination is relatively high. Third, there is a practical limitation to the gap that can be formed by the bursting bridge. Specifically, only so much chemical charge may be confined within a practical volume of the bursting bridge to ensure that the fingers defined by the slots in the two tube sections are sufficiently blown outwardly and bent backwardly. That is, the tube sections could be greatly elongated and filled with a chemical charge of larger size so that its detonation bends back fingers of increased length to produce a longer gap. Both the increased size of the charge and the increased length of the fingers, however, require a larger diameter housing of higher burst-strength, adding to the cost and inconvenience of the overall device. Fourth, as already noted, some rather precise coordination between the operation of the current-limiting fuse of the I.sub.s -Limiter and the dielectric recovery of the gap formed between the tube sections is necessary. Due to the vagaries of fault-current conditions in high-voltage circuits, this coordination may prove difficult to achieve.
A complete discussion of the I.sub.s -Limiter may be found in the following documents: "A Current-Limiting Device for Service Voltages Up to 34.5 kv" by Keders and Leibold, Paper A76 436-6, presented at the IEEE PES Summer Meeting, Portland, Oreg., July 18-23, 1976; "Limiting Fault Currents Between Private and Public Networks" by Blythe, The Electrical Review (Great Britain), Oct. 5, 1973; "Fault Levels Too High?" an English Language publication put out by Calor-Emag Corporation as Leaflet No. 1197/6E; "The Application of I.sub.s -Limiters in Three-Phase Systems" by Bootger, a publication of the Calor-Emag Corporation, circa August 1967; and "The Economic Benefits of Using I.sub.s -Limiters" by Heilmann, a publication of the Calor-Emag Corporation, circa February 1963.
Other types of circuit interrupters utilizing the blowing apart of a conductor by an explosive charge are disclosed in the following: U.S. Pat. Nos. 466,761 to Wotton; 1,856,701 to Gerdien; 2,175,250 to Burrows et al; 2,548,112 to Kaminky; 2,551,858 to Stoelting et al; 3,400,301 to Misare; 3,851,210 to Kozorezov et al; 3,958,206 to Klint; and French Pat. No. 2,262,393 to Grebert.
Some general improvement of devices similar to the I.sub.s -Limiter has been effected, as described by Pflanz, Clark, and Laboni, in "A New Approach to High-Speed Current Limitation," presented in the Symposium Proceedings, New Concepts in Fault-Current Limiters and Power Circuit Breakers, printed in a special report of the Electrical Power Research Institute, Paper EPRI EL-276-SR, in April 1977.
In the Pflanz et al device, a fusible element is embedded in and surrounded by a fulgurite-forming particulate medium, such as silica sand, to form a current-limiting fuse apparently of more or less standard design. The fusible element is electrically paralleled with a large-cross-section copper conductor which constitutes a main current-path. The fusible element and the conductor are contained in a common insulative housing. The large-cross-section conductor is surrounded by, and has wound around it, a so-called "linear charge" which, upon detonation, cuts through the large-cross-section conductor to create a plurality of gaps therein. The formation of these gaps commutates the current normally flowing through the conductor to the fusible element for current-limiting interruption of a fault current. Detonation of the linear charge is initiated by a sensor/initiator, which is described only as a "fuse primary charge," responsive to either current flowing through the large-cross-section conductor, or to the output of a current transformer. According to Pflanz, et al, the sensor and initiator may be either contained within the common housing for the device or externally thereof. As should be apparent, the Pflanz, et al, device operates substantially externally the same as the I.sub.s -Limiter except that plural gaps are formed in the main current-conductor prior to current-limiting circuit interruption by the current-limiting fuse. The Pflanz, et al, device suffers at least two of the shortcomings of the I.sub.s -Limiter. Specifically, although numerous gaps are formed in the main conductive path, the length of these gaps is nevertheless limited by the ability of the linear charge to render the large-cross-section conductor discontinuous. There is a practical limit to the dimensions these gaps may achieve; apparently the gap dimensions are quite small. Thus, it would seem that the possibility exists for restriking of arcs in the small gaps, should the arc voltage in the current-limiting fuse reach high levels. Second, although the Pflanz, et al, device decreases the inductance of the overall device, as compared to the I.sub.s -Limiter, by placing the fusible element and the main conductive path in the same housing, reduction of such inductance has not been optimized.
Other devices related to the I.sub.s -Limiter and to the Pflanz, et al, device, either by their use of chemical charges or by their parallel arrangement of current paths, are also known. A summary follows.
It is known to ignite or detonate a chemical charge with heat caused by a fault current, the exothermic ignition of the charge melting or breaking a member. The member normally restrains movement of an element; melting or breaking of the member permits a stored energy source or spring to perform work, such as moving the element to operate a circuit breaker operating lever. See U.S. Pat. No. 1,917,315 to Biermanns et al.
It is broadly known to move a contact and close a circuit by the detonation of a chemical charge. In U.S. Pat. No. 3,184,726 to Hellgren, detonation of a pyrotechnic mixture pressurizes a housing. The end of a bellows forming a part of the housing is moved by such pressurization. The bellows end ultimately engages a grounded contact to ground a circuit which includes the housing therein.
In U.S. Pat. No. 2,721,240, to Filbert, detonation of an explosive charge everts or deforms a ductile, conductive diaphragm. Eversion or deformation of the diaphragm causes it to engage and electrically interconnect a pair of separated contacts, thus completing a circuit therebetween.
Perry and Frey in an article entitled "Ultra-High Speed Ground Switch Application and Development" (AIEE Paper No. 62-1109, presented in Denver, Colo., in June 1962) describe a ground switch having a lightweight blade (e.g., aluminum tubing) connected to a piston of a piston-cylinder. The cylinder contains an electrically firable propellant cartridge, the firing circuit for which contains a normally open switch. When a sensor detects a predetermined condition in a high-voltage system, the normally open switch is closed to fire the cartridge. Firing of the cartridge pressurizes the piston-cylinder to rapidly move the piston. Rapid piston movement rapidly pivots the blade on an electrically grounded hinge into engagement with a mating contact connected to the high-voltage system. The system is thus grounded.
McMorris, U.S. Pat. No. 2,305,436, described a fuse device, which includes a fusible element in electrical series with an inductor, the series combination being in electrical parallel with a spark gap. The fusible element is surrounded by an explosive charge (e.g., gunpowder) contained within a cardboard housing. The inductor physically surrounds the spark gap and the cardboard housing. One side of the fusible element is electrically and physically connected to one electrode of the spark gap. Acting between the one electrode and a terminal of the device is a spring, which also is a current path between the one electrode and the terminal. All elements are in an insulative housing closed by a porcelain disk cemented thereto near the terminal. If the device is subjected to a prolonged surge, the spark gap first breaks down and conducts because of the voltage developed across the inductor. Subsequently, the gap ceases conduction and current flows through the inductor and the fusible element, blowing the fusible element to detonate the explosive charge. Detonation of the charge fractures the cement joint between the disk and the insulative housing, permitting the spring to expel the terminal from the housing.
In U.S. Pat. No. 1,917,315 to Murray, a high tension fuse includes a hollow tube having a pair of low mass plungers therewithin. It is not clear if the plungers are insulative or conductive. A fusible element runs the length of the tube through the plungers and has a "blowing point" between the plungers. A quantity of gun cotton may be on one of the plungers near the blowing point. When a fault current occurs in a circuit to which the fusible element is connected, gas generated by the fusing of the blowing point, and by detonation of the gun cotton effected by such fusing, drives the plungers apart. The plungers carry with them portions of the fusible element passing therethrough.
Curry, in U.S. Pat. No. 2,491,956, discloses a circuit interrupter having a high resistance path in electrical shunt with a low resistance path. The low resistance path includes, in series, a terminal, a bimetallic element, a first movable contact on the element, a second movable contact normally engaged by the first movable contact, a movable contact rod mounting the second movable contact, and a sliding contact continuously electrically connected to the contact rod. The contact rod and the second movable contact are biased for movement away from the first movable contact by a spring. This bias is normally resisted by a fusible strain wire. The high resistance path includes, in series, the terminal, the strain wire, a portion of the contact rod, and the sliding contact. Excessive current flow through the interrupter heats the bimetallic element, causing it to flex and disengage the first movable contact from the second movable contact. This, in turn, transfers the current to the strain wire, which fuses, permitting the spring to move the contact rod and the second movable contact away from the first movable contact. Such movement elongates the arc between the movable contacts in an arc-extinguishing environment to interrupt the excessive current.
None of the above references disclosed devices intended for current-limiting circuit interruption. Moreover, some of them (Hellgren, Filbert, and Perry and Frey) are either low-voltage devices or are "close only" switches or grounding switches. In Biermanns, et al, only the heat energy of a chemical charge is utilized; in McMorris, detonation of an explosive charge is primarily utilized to disintegrate a housing so that a spring may expel a terminal; Curry uses no chemical charge or explosive at all. In Murray, the electrical connection between two plungers is first broken, following which the plungers move apart. As will soon be apparent, the present invention involves, in part, movement apart of two contacts, following the inception of which movement, normal electrical interconnection therebetween is broken by the movement. Lastly, all of these prior art devices are complicated, are unsuitable for high-voltage circuit interruption, are of doubtful operability, or all of these.
The invention disclosed in commonly assigned U.S. patent application, Ser. No. 972,650, filed Dec. 21, 1978 in the name of Otto Meister is an improvement over all of the previously discussed devices. Specifically, an improved high-voltage device having a high continuous-current rating, may include both a fuse and an improved switch. The device has a first, high-current-capacity path and a second, low-current-capacity path surrounding the first path in a compact configuration. Current is selectively commutated from the first path, which may include the switch, to the second path, which may include the fuse. The improved switch has a pair of normally electrically interconnected contacts. The contacts are relatively movable apart along a fixed line of direction to break the electrical interconnection. The contacts define an enclosed chamber. The chamber may be pressurized by ignition of a power cartridge therein to rapidly drive and move the contacts apart. Preferably the improved device comprises a current-limiting fuse which helically, coaxially surrounds the improved switch in a common housing.
The device and switch of the '650 application do not depend upon the mere fracturing (or blowing apart) and peeling back of portions of a main current path, as is the case with some prior art devices, but rather, utilize the positive driving and moving apart of the contacts, ensuring that a large gap is opened therebetween. The surrounding relationship of the current paths not only decreases to a minimum the inductance of the overall device, but further, minimizes the number of directional changes which the commutated or transferred current experiences, keeping the current flowing in the same direction in the second current path as it flowed in the first current path. Further, the surrounding relationship renders the fuse convenient to fabricate and assemble.
While the invention of the '650 application represents an improvement over the above-discussed prior art devices, it is recognized that refinements thereof are possible, and, perhaps, desirable. For example, although contact movement apart permits the formation of a long gap, hot ignition products of the power cartridge may permit or encourage arcing therebetween to persist and/or lower the arc voltage. Both effects may slow or prevent commutation of the current to the second current path; means of obviating this result--suppressing or extinguishing the arc, elevating the arc voltage, or opening additional gaps in the first current path--are only generally suggested in the '650 application.
Moreover, im the invention of the '650 application, the enclosed chamber is defined by the contacts which have blind holes therein. The contacts normally engage along an annular interface, the blind holes defining the enclosed chamber. The interface may include a conductive medium, or be brazed or soldered, to ensure normal electrical conduction between the contacts. Such conduction may also be ensured by only generally described breakable, tearable or frangible conductive members. The intent of the '650 invention is that pressurization of the chamber by the power cartridge separates the contacts along the interface, or breaks the conductive members, where used. In devices according to the '650 application the interface may not always predictably separate.
Accordingly, a primary object of the present invention is to improve and refine the '650 invention, including the following objects:
(1) To positively ameliorate or obviate the effects of the ignition products of the power cartridge; PA1 (2) To ensure current commutation to the second current path by suppressing or extinguishing any arc forming, or tending to form, between the separated contacts; PA1 (3) To restructure the annular interface between the contacts--eliminating the need for the conductive medium, soldering or brazing--and both ensure that the contacts can rapidly move apart in a predictable fashion, while eliminating the need for the interface to carry current between the contacts; and PA1 (4) To restructure the chamber so that its pressurization more effectively drives the contacts apart.
A further object of the present invention is the provision of a high-voltage fuse having a high continuous current rating. Yet another object of the present invention is a high-voltage fuse having the following properties: convenient, expeditious and economical manufacture; reliable operation; simplification and minimization of parts; minimization of inductance; and reliable formation of a gap in a main conductive path which ensures current commutation to a fusible element. An additional object of the present invention is the provision of a switch for use in the main conductive path, in which switch a pair of normally electrically interconnected contacts are moved apart by ignition of a charge in a chamber, the movement breaking the electrical interconnection therebetween.